Method for reconstructing a video frame received from a video source over a communication channel

ABSTRACT

An adaptive region-based, multi-scale, motion compensated video compression algorithm design for transmission over hostile communication channels. The algorithm is embodied in a video encoder that extracts spatial information from video frames to create video regions that are then decomposed into sub-bands of different perceptual importance before being compressed and transmitted independently. The system further uses unequal error protection, prioritized transmission and reconstruction to guarantee a minimum spatial and temporal resolution at the receiver. In particular, the region segmented frames bound both spatial and temporal error propagation within frames. A connection-level inter-region statistical multiplexing scheme is also employed to ensure optimal utilization of reserved transmission bandwidth.

FIELD OF THE INVENTION

This invention relates generally to communication networks and, moreparticularly, relates to a video encoder for generating informationrepresentative of a video frame, a method for transmitting the generatedinformation over a communication channel, and a method forreconstructing the video image from the generated information.

BACKGROUND OF THE INVENTION

Video communications over a communication network are currently beingcarried out using source encoders and channel encoders that weredesigned independently of each other. The tradition of separating thesource and channel encoding tasks arises from the theoretical foundationof Shannon's separation principle of source and channel encoding thatespouses the notion that this separation is optimal. According to thisprinciple, the task of the source encoder is to do the best job it canin compressing the input signal so that the compressed bit rate willmatch the available bit rate of the communication channel.

Shannon's separation principal does, however, fail to address the errorcharacteristics of the communication channel on which the videoinformation will traverse. Importantly, developing an encoder withoutregard to these error characteristics has lead to the result that theimage signal is rendered effectively useless if even a few bits arecorrupted during transmission. This problem becomes especiallypronounced in a wireless communication network where large variationsand unpredictability in the error characteristics abound. Unfortunately,currently utilized video coding standards including H.261, MPEG-1,MPEG-2 and H.263, which were developed without regard to the errorcharacteristics of the communication channel, fail to provide the errorresiliency needed to minimize this problem.

Furthermore, currently utilized error protection and error recoveryschemes that are employed to alleviate the noted error-induction problemhave the undesirable side effect of wasting bandwidth within thecommunication channel. For example, a commensurate amount of bandwidthmust be allocated to video transmissions at the expense of othertransmissions when additional error checking bits are added to the videobit-stream. Furthermore, using an automatic repeat request (ARQ)procedure that functions to initiate the re-transmission of thosetransmissions deemed to be in error, while providing error recovery,adds both delay and bandwidth overhead to the transmissions. Delay forthe case of interactive real-time video applications causes unacceptableapplication performance, while bandwidth overhead reduces the efficiencyof the overall system.

From the foregoing, it is evident that a need exists for software andhardware mechanisms that ensure timely and robust delivery of video dataover bandwidth-limited, error-prone communication channels.

SUMMARY OF THE INVENTION

In accordance with this need, the subject invention may be generallydescribed as a combined application and network level solution forachieving robust video transmission over bandwidth-limited anderror-prone communication channels. More specifically, the desired videotransmission is achieved through the use of a method for generatinginformation indicative of a video frame, a method for transmitting thegenerated information over the communication channel, and a method forreconstructing the video information from the generated information.

For generating the information indicative of a video frame, the videoframe is segmented into a plurality of discrete spatial components andeach of the plurality of discrete spatial components is transformed intoa plurality of discrete frequency components. For transmitting thegenerated information over the communication channel, the videocomponents are evaluated to determine the peak number of bits requiredfor their transmission, bandwidth is requested as a function of thisdetermination, the most important video components are first transmittedin the requested bandwidth and any remaining bandwidth is used totransmit any remaining video components. For reconstructing the videoinformation from the generated information the video components receivedare checked for transmission errors and, if any such errors are present,video components from a previous transmission are substituted therefor.In this manner, the invention overcomes the noted deficiencies in theprior art by, among other things, reducing the reliance on errorcorrection and error recovery schemes that are traditionally used intransmission over error-prone channels and preventing bandwidth wastagethough intelligent bandwidth reservation and utilization whileguaranteeing the promised spatial and temporal video resolution at thereceiver.

A better understanding of the objects, advantages, features, propertiesand relationships of the invention will be obtained from the followingdetailed description and accompanying drawings which set forth anillustrative embodiment which is indicative of the various ways in whichthe principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to apreferred embodiment shown in the accompanying drawings in which:

FIG. 1 illustrates a block diagram of an exemplary communication networkin which subject invention may reside;

FIG. 2 illustrates he exemplary network of FIG. 1 in the form of acellular communication network;

FIG. 3 illustrates a further block diagram of a communication source,network router, and communication receiver that reside in thecommunication network of FIGS. 1 and 2;

FIG. 4 illustrates a flow chart diagram depicting a method forgenerating information indicative of a video frame performed inaccordance with an embodiment of the subject invention;

FIG. 5 illustrates a block diagram of an embodiment of a video encoderthat functions to perform the method illustrated in FIG. 4;

FIG. 6 illustrates two pictures of a video frame that has been spatiallysegmented in accordance with the method illustrated in FIG. 4;

FIG. 7 illustrates two pictures of a video frame for the purpose ofdemonstrating the before and after effects of performing athreshold-based spatial segmentation on the difference image obtained bysubtracting two successive video frames as done in the motion estimationand compensation component of the subject invention;

FIG. 8 illustrates the steps and corresponding results when segmenting avideo frame in the frequency domain in accordance with the methodillustrated in FIG. 4;

FIG. 9 illustrates a block diagram of a package for use in transmittingthe information indicative of the video frame generated in accordancewith the method illustrated in FIG. 4;

FIG. 10 is a flow chart diagram illustrating a method for reservingbandwidth and subsequently for transmitting the information indicativeof the video frame over he communication channel;

FIG. 11 illustrates two graphs for the purpose of demonstratingbandwidth demand versus bandwidth usage before and after using themethod illustrated in FIG. 10;

FIG. 12 illustrates the comparative effects of error propagation on avideo frame with segmentation, with spatial segmentation, and with bothspatial and frequency segmentation performed according to the methodillustrated in FIG. 4;

FIG. 13 illustrates a flow chart diagram depicting a method forreconstructing a video frame at the receiver; and

FIG. 14 illustrates the movement of video frame segments when performingthe frame reconstruction method illustrated in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings, wherein like reference numerals refer to likeelements, the invention is illustrated as being implemented in asuitable network environment. Although not required, the invention willbe described in the general context of computer-executable instructions,such as program modules, being executed by a processing device such as apersonal computer, mainframe computer, or the like. Generally, programmodules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Moreover, those skilled in the art will appreciatethat the invention may be practiced with other processing devices suchas consumer electronic devices having one or more processors such as,for example, mobile telephones. The invention may also be practiced indistributed computing environments where tasks are performed by remoteprocessing devices that are linked through a communication network andwhere program modules are located in both local and remote memorystorage devices.

With reference to FIG. 1, an exemplary network system in which theinvention may reside is illustrated. The network system includes acommunication source 20 illustrated in the exemplary form of a personalcomputer. The communication source 20 includes a processing unit 21, asystem memory 22, and a system bus 23. The system bus 23 functions tocouple the various system components including the system memory 22 tothe processing unit 21. The system bus 23 may be any of several types ofbus structures including a memory bus or memory controller, a peripheralbus, and a local bus using any of a variety of bus architectures. Thesystem memory 22 includes read only memory (ROM) 24 and random accessmemory (RAM) 25. A basic input/output system (BIOS) 26, containing thebasic routines that help to transfer information between elements withinthe communication source 20, such as during start-up, is stored in ROM24. The communication source 20 may also include a hard disk drive 27, amagnetic disk drive 28, or an optical disk drive 30. It will beappreciated that these devices respectively allow for reading from andwriting to a hard disk, reading from or writing to a removable magneticdisk 29 and for reading from or writing to a removable optical disk 31,such as a CD ROM or other optical media.

When incorporated into the communication source 20, the hard disk drive27, magnetic disk drive 28, and optical disk drive 30 are connected tothe system bus 23 by a hard disk drive interface 32, a magnetic diskdrive interface 33, and an optical disk drive interface 34,respectively. The drives and their associated computer-readable mediaprovide nonvolatile storage of computer readable instructions, datastructures, program modules and other data for the communication source20. It will be appreciated by those skilled in the art that other typesof computer readable media that can store data may also be used.Examples of such media devices include, but are not limited to, magneticcassettes, flash memory cards, digital videodisks, Bernoulli cartridges,random access memories, and read only memories.

A number of program modules may be stored in one or more of the memorydevices and typically include an operating system 35, one or moreapplications programs 36, other program modules 37, and program data 38.A user may enter commands and information into the communication source20 through input devices such as a keyboard 40 and pointing device 42.Other input devices (not shown) may include a microphone, joystick, gamepad, satellite dish, scanner, or the like. These and other input devicesare often connected to the processing unit 21 through a serial portinterface 46 that is coupled to the system bus, but may be connected byother interfaces, such as a parallel port, game port or a universalserial bus (USB). A monitor 47 or other type of display device is alsoconnected to the system bus 23 via an interface, such as a video adapter48. In addition to the monitor 47, the communication source 20 may alsoinclude other peripheral output devices, not shown, such as speakers andprinters.

The communication source 20 operates in a networked environment usinglogical connections to one or more remote communication receivers 49,also illustrated in the exemplary form of a personal computer. Theconnection is typically made through a further processing device 100that is responsible for network routing. In the illustrated embodiment,the remote communication receiver 49 will include many or all of theelements described above relative to the communication source 20including the memory storage devices and monitor 47. Furthermore, withinsuch a networked environment, it will be appreciated that programmodules depicted relative to the communication source 20, or portionsthereof, may be stored in the memory storage devices of the remotecommunication receiver 49.

The description that follows will refer to acts and symbolicrepresentations of operations that are performed by the processingdevices 20,49,100 unless indicated otherwise. As such, it will beunderstood that such acts and operations, which are at times referred toas being computer-executed, include the manipulation by the CPUs of thedevices of electrical signals representing data in a structured form.This manipulation transforms the data or maintains it at locations inthe memory system, which reconfigures or otherwise alters the operationof the processing devices 20,49,100 in a manner well understood by thoseof skill in the art of computer systems. The data structures where datais maintained are physical locations of the memory that have particularproperties defined by the format of the data. While the invention isbeing described in the foregoing context, it is not meant to be limitingas those skilled in the art will further appreciate that various of theacts and operation described herein may also be implemented in hardware.

In a preferred embodiment, the network environment comprises a wireless,cellular communication network such as depicted in FIG. 2. Nevertheless,while the invention will be described hereinafter in that context, thoseof skill in the art will appreciate how to adapt the features of thesubject invention for use in other communication networks such as alocal area network (LAN) or a wide area network (WAN). Accordingly, thedescription that follows is not intended to be limiting.

Turning to FIG. 2, a cellular communication network having architecturethat is similar to the architecture that is found in a Global System forMobility (GSM) network is illustrated. As will be well understood bythose of skill in the art, mobile terminals MT communicate over a radiochannel with a network router in the form of a base transceiver stationBTS that is located within the same cell. The base transceiver stationsBTSs of neighboring cells communicate over high speed wired or wirelesschannels, with a base station controller BSC that serves as theirmanager. In turn, the base station controllers BSCs in a geographicregion communicate over high speed wired or wireless channels with amobile switching center MSC that serves as their manager. The mobileswitching centers MSCs are connected to a public network that mayinclude public switched telephone networks PSTNs, integrated servicesdevice networks ISDNs, or asynchronous transport mode ATM networks.

More specifically, the mobile terminals MT within the network areenvisioned to be communication sources 20 of the following types: 1)communication sources 20VO performing voice packet transmissions; 2)communication sources 20DA performing data packet transmissions; and 3)communication sources 20VI performing video packet transmission.Meanwhile, linked to the public network as a destination for thetransmissions originating from the mobile terminals MT are remotecommunication receivers 49 of the following type: 1) remotecommunication receivers 49VO receiving voice transmissions; 2) remotecommunication receivers 49DA receiving data transmissions; and 3) remotecommunication receivers 49VI receiving video transmissions. It will beappreciated by those skilled in the art that the remote communicationreceivers 49 may themselves be mobile.

In accordance with one important aspect of the invention, thecommunication network provides for the efficient transmission of videocommunications from the communication sources 20VI to one or moreassociated remote communication receivers 49VI for subsequent display toremote users. For this purpose, as illustrated in FIG. 3, thecommunication sources 20VI are each provided with a video camera 152that functions to capture video images as a series of digital videoframes. Within the communication sources 20VI, the video camera 152 isconnected to a video encoder 154 via the system bus 23. The videoencoder 154 functions to generate information that is representative ofthe video frame which is subsequently supplied to a transmitter 155 forbroadcasting over the communication channel to the network router 100,e.g., base station BS. Meanwhile, the remote communication receivers49VI each include a video decoder 156 that functions to recreate thevideo frame from the information received over the communicationnetwork. The decoded video frame may then be forwarded to an associatedmonitor 47 for viewing through the use of an appropriate video adapter157. For a description of the communications protocol that is preferablyused in broadcasting the video information over the communicationnetwork the reader is referred to commonly owned, co-pending U.S. patentapplication No. 09/169,418 entitled Channel Access Scheme For Use InNetwork Communications that is incorporated herein by reference in itsentirety.

Turning to FIGS. 4 and 5, the video encoder 154 functions to generateinformation representative of a video frame for use in providing visualcommunications over a communication network. Generally, the videoencoding algorithm utilized within the encoder 154 generates theinformation by decomposing individual frames of a video sequence into aplurality of discrete, generally homogeneous video regions by performingsegmentation in both the spatial and frequency domains. For thispurpose, the encoder includes a spatial segmenter 160 and a frequencysegmenter 162. Additionally, the video encoder 154 includes a frameclassifier 158 and a compressor 164 comprised of a quantizer 166 and runlength encoder 168. With the exception of the spatial segmenter 160 andthe frequency segmenter 162, the components of the encoder 154 aresimilar to those found within an encoder that currently utilizes theMPEG-1 coding standard.

The frame classifier 158 is provided to classify the image frames, orsegments thereof, as either an inter-frame (when it is to be coded withrespect to other frames or segments) or an intra-frame (when it is to becoded independently). This classification is typically based on theorder and motion activity of the video frame. For example, the firstvideo frame or anchor frame of the video sequence or frames containingdrastic scene changes are intra-coded while the remaining frame areintercoded. Additionally, every n^(th) video frame may be encoded as anintra-frame, where n is some predetermined number, in order to minimizeinter-frame error propagation that may arise from time to time.

The spatial segmenter 160 is provided to create a spatial segmentationmap for the video frame. Generally, for intra-frames, the entire imageis segmented and a segmentation map is derived, for inter-frames,segmenting is employed and a segmentation map is derived only when thereis significant motion activity within the video frame sequence. When themotion activity is below a predetermined threshold (a design variable),the previous most recently derived segmentation map is reused. When themotion activity is above the threshold, a new segmentation map isderived and forwarded to the remote computer 40, typically with theencoded frame information in a transmission packet. As will be betterunderstood from the description that follows, the communication receiver49VI requires the segmentation map in order to recreate the video imagesfor display. An advantage to basing the segmentation decision on motionactivity is found in the significant reduction in overhead owing to thefact that a new segmentation map does not have to be forwarded to thecommunication receiver 49VI with every frame. Accordingly, the encoderand the decoder are designed to continue to use the same segmentationmap until some significant scene change appears in the incoming frame atwhich time a new segmentation map is generated from the statistics ofthat frame.

To dynamically generate the segmentation map for those image frames thatare independently coded, i.e., intra-frames, the spatial segmenter 160first preferably partitions the digital image frame into blocks of size16×16 pixels. For each block the variance of pixel intensity is thencalculated. Adjacent blocks of similar variances that are within apredetermined threshold are then merged. The merged blocks form thespatial segmentation map examples of which are illustrated in FIG. 6.

Specifically, the blocks are merged when the following criterion issatisfied:

|log(σ_(i) ²/σ_(j) ²)|<T   (1)

wherein σ_(i) ² and σ_(j) ² are the variances of two adjacent blocks iand j in a video frame that is being segmented and T is a thresholdvalue for controlling the decomposition of the image frame. Inparticular, T may be defined by the following equation

T=log(σ_(max) ²/σ_(min) ²)/N.   (2)

wherein σ_(max) ² and σ_(min) ² are the maximum and minimum value,respectively, of the variances of all of the blocks within the imageframe and N is the total number of blocks within the image frame. In afurther embodiment of the invention, the value of the threshold T isadditionally biased upward for the purpose of more clearly separatingvideo objects from the background. Accordingly, T may be defined by thefollowing equation

T=log(σ_(max) ²/σ_(min) ²)/N+η  (3)

wherein η is the mean value of the pixel intensity of all of the blocksin the video frame.

At this time, it is noted that there exists a trade-off betweencomplexity and coding performance when choosing the block size. Whileusing smaller sized blocks achieves relatively better spatialdecomposition, the overhead incurred for sending the segmentation map tothe decoder is relatively higher. For this reason, the block size of16×16 was selected as it was seen to yield relatively good adaptabilitywith acceptable levels of overhead for each intra-frame image.

As described, for inter-frames, segmentation is employed only when thereis significant motion activity within the video frame sequence. For thispurpose, the video encoder 154 further includes a motionestimator/compensator 170 that functions to determine motion differencesbetween two sequential image frames. To perform this task, theestimator/compensator 170 segments the image frame into 16×16 blocks andutilizes a matching algorithm to obtain motion vectors. The matchingalgorithm is employed to see if any of the blocks within the currentimage frame is sufficiently similar to a block within the previous imageframe. During this search, it is preferred that the absolute error foreach pixel as well as for each block be calculated to determine if theblocks are similar within a predetermined threshold. Exemplary resultsachieved as a result of performing these steps may be seen in FIG. 7.

If the motion difference between the two images is sufficient, theaffected blocks are preferably classified as intra-frame blocks and anew segmentation map for these blocks is generated using the spatialsegmenter 160 and the related methodology described previously. Themotion difference between the images is typically deemed to besufficient if: 1) any blocks have moved greater than a predetermineddistance within the images; or 2) any blocks are determined to have nogeneral match with any block in the previous image frame. The lattercondition is utilized for the reason that it signifies that a new actionobject has likely entered the video sequence. In a further embodiment,it is preferred to reclassify a video frame as an intra-frame when themotion difference, i.e., the number of affected blocks, is above somemaximum value. In this case, a completely new segmentation map for theentire frame would be generated using the spatial segmenter 160.

In yet another embodiment of the invention, the spatial segmenter 160uses a predefined segmentation map to merge the blocks without regard tothe intra-frame or inter-frame considerations set forth above. Thispredefined segmentation map is preferably arranged such that segmentsoverlaying regions of less visual importance within the video framesequence, such as background, contain a large number of merged blockswhile segments overlaying areas of more visual importance, such asaction objects, contain fewer merged blocks. Accordingly, it will beappreciated that the type of video to be transmitted should dictate theform of the predetermined segmentation map that is utilized. As will beappreciated, this method for segmenting the video frame has theadvantage of requiring a lesser amount of computational energy whencompared to the dynamic segmentation method described previously. As aresult, the use of a static segmentation map has the advantage ofproviding a means for conserving power within the communication source20VI. In addition, this method also has the advantage of providing ameans to conserve bandwidth as the segmentation map needs to betransmitted to the communication receivers 49VI only one time owing toits static nature.

When establishing a segmentation map, it is also preferred that thespatial segments be provided with a transmission priority. Thistransmission priority is preferably established as a function of theinformation contained within the various segments. For example, largersegments that would typically contain background information would begiven a lower transmission priority while smaller segments that wouldtypically contain action information would be given a highertransmission priority.

Once the image frame has been spatially segmented, each of the spatialsegments is further decomposed into a plurality of discrete frequencycomponents as illustrated in FIG. 8. For this purpose, the frequencysegmenter 162 preferably utilizes a known wavelet-based technique thatfunctions to represent an image signal at different levels ofresolution. Generally, wavelets, when used in sub-band coding, emphasizeboth spatial localization and frequency localization during the bandsplitting procedure. In this manner, wavelet transformation provides formulti-resolution decomposition of the image frame. The image frame isthus represented by compact, non-redundant and exact representations atdifferent scales.

More specifically, the frequency segmenter 162 is preferably embodied ina two-tap Harr filter that functions to decompose the luminance (Y)components of each segment into four bands, namely, one DC-subband(low-low band) and three AC-subbands (low-high, high-low, and high-highbands). The Haar filter was choosen because of its low computationalcost and capability to decompose the arbitrarily shaped regionsgenerated by the spatial segmenter 160 without causing any undesirableboundary effects. Following the wavelet decomposition, the frequencysegmenter 162 also preferably applies an 8×8 Discrete Cosine Transform(DCT) in a moving window to each of the subbands. The DCT functions tofurther decorrelate the spectral components and causes energy compactionin coefficients. However, the DCT transformation step can be skipped ifa more elaborate wavelet transformation is used such as a Daubechies6-tap filter.

As described, the compressor applying the Harr transformation followedby the DCT to the image frame segments de-correlates the signals of theoriginal image and concentrates the image information into a relativelysmall number of coefficients. Since most images consist primarily ofsmooth or low frequency components, it is often true that the DC-subbandcontains most of the energy of the image. As the DC-subband provides asmoothed and downsampled version of the original image while theAC-subbands provide information of edges and textures, the DC-subbandsof each of the spatial segments should be given a still higher priorityfor transmission than should be given to the AC-subbands.

For the purpose of conserving communication channel bandwidth,information that comprises each of the subbands for each of the segmentsis preferably further compressed before it is transmitted over thecommunications network. Accordingly, a compressor 164 in the form of aquantizer 166 and standard run length encoder 168 is provided.Preferably, the quantizer 166 maximizes this compression by eliminatingall but a few coefficients which still allows a good quality image to bereconstructed. This is possible owing to the above-noted observationthat most of the image may be found within the DC-subband. Morespecifically, the quantizer 166 attempts to maximize compression byeliminating those coefficients below a user specified threshold and byapplying a different quantizer to each segment as a function of itspriority. Within these regions, the coefficients of the DC-subband arefurther quantized with a lower step size than are the AC-subbands. Inthis manner, the more important information of the image is preserved.In particular, in order to preserve the visual quality of anintra-frame, all coefficients of the lowest frequency sub-bands arepreferred to be protected for all video regions. For inter-frame, it ispreferred that all motion information be protected.

Turning to FIG. 9, the package that is used to transmit the informationindicative of the video image is illustrated. Generally, the packageincludes a picture header, a segment map, and a segment data layer. Thepicture header is similar to the picture header utilized in the H.263video coding standard and contains all information needed to decode onevideo frame such as a 20-bit start code (PSC), a 5-bit temporalreference (TR) or frame number, and type information (PTYPE). Inaccordance with this invention, the header also contains a segmentidentifier (SI) and a bit-field representing the total number ofsegments (NS) within the picture. As a means to further conservebandwidth, the segment identifier (SI) was selected to be a four-bitfield. Accordingly, as each segment within the segmentation map would berepresented by a unique 4 bit segment index, the spatial segmentationmethods described above will have to be adapted to ensure that thenumber of discrete spatial segments that the video frame is divided intodoes not exceed sixteen. Within the segment map, the first bit ispreferably utilized to inform the communication receiver 49VI that thesegment map contained within the package is new.

Within the segment data layer, the DC subbands and the AC subbands foreach video frame are transmitted in different slices. Preferably theslices are arranged such that the DC subbands first appear in theirspatial segmentation transmission priority order followed by thecorresponding AC subbands similarly ordered. This structure allows thesystem to apply unequal error protection (UEP) and transmit the videobased on the transmission priority of the segments and the subbands.

Specifically, the segmentation approach described above facilitatesguaranteed quality of service during the transmission of the videoinformation by generally reserving bandwidth as a function of thetransmission priority of the subbands of the video segments.Specifically, the number of bits required for transmitting the DCsubbands of all the video segments within a video frame is determined.This procedure is repeated for all the video frames within the videosequence that is to be transmitted. Then the maximum (peak) of all thesenumbers is multiplied by the desired target video frame display rate andthe result determines the amount of bandwidth that is to be staticallyreserved at the connection establishment time. If the reservation issuccessful, it ensures that there will always be bandwidth available fortransmitting at least the DC subbands of all the video frames in thevideo sequence. This guarantee when combined with the video framereconstruction procedure, to be described next, guarantees that aminimum video frame temporal resolution can be supported at thereceiver. This procedure is illustrated in FIG. 10. As it is likely thatmost of the time the compressor 164 will produce bits far below thispeak number, as illustrated in FIG. 11, the notion of statisticalmultiplexing within a frame is introduced to avoid under-utilizing theallocated bandwidth. With statistical multiplexing, any bandwidthleftover after the DC subbands have been transmitted is used fortransmitting the remaining AC subbands. Also, packets whoseretransmission has been requested by a communication receiver 49VI canbe sent using the left over bandwidth that was reserved. Furthermore,additional bandwidth can be requested dynamically for the purpose oftransmitting any packets that remain after all of the staticallyallocated bandwidth has been used. In this manner, optimum bandwidthutilization may be achieved.

At the receiver, transmission errors due to channel imperfections cancause corruption in some of the transmitted video segments renderingthem un-decodable. For example, FIG. 12 illustrates error propagationwith and without spatial segmentation as a result of a video bit streamthat has been corrupted with errors not caught by an error detector. Adynamic reduction in non-reserved bandwidth whereby some of the segmentsfail to reach the decoder in a timely manner will result in the receivernot being able to reconstruct the complete image. Accordingly, tominimize any errors and be able to create a complete image fromincomplete data that may arise if either of these events occur, thereceiver employs a frame reconstructing algorithm, illustrated in FIG.12, that utilizes a combination of the current and previouslytransmitted video segments that were correctly received.

Specifically, if S_(ij) represents the video segment j in frame i, thenif all video segments are received error-free, the complete frame i isformed as:

S_(i)=ΣS_(ij)   (4)

In the case when some of the S_(ij) are incorrectly received, S_(i) isformed using the last corresponding j^(th) video segment that wasreceived correctly. This methodology of image reconstruction at thereceiver is generally illustrated in FIG. 14 for a simple horizontalpartitioning scheme.

As will be understood from the foregoing discussion, the video segmentsstored in the receiver's region store may not belong to the same videoframe. Accordingly, when used to reconstruct the video frame in thismanner, the temporal differences between the current video segment andthe previously correctly received corresponding video segment dictateshow good or bad the final picture looks. When this difference is large,visual quality is impaired by the tearing effect. However, when theprevious good video segment is from a preceding sequential frame, thequality tends to be visually acceptable. Therefore, to reduce thetearing effect, the communication receiver 49VI may demand from thecommunication source 20VI the immediate transmission of the videosegment(s) that it had to substitute in order to reconstruct the currentframe. The communication source 20VI may then switch the transmissionpriority of the requested segment(s) to ensure that the requested videosegment(s) definitely reach the communication receiver 20VI. With such ascheme, the difference between the current video segment and the onesstored in the receiver's region store is never too much and the tearingeffect is mitigated.

From the foregoing descriptions, it will be appreciated that theinvention described herein provides a solution to the problem ofguaranteeing a temporal and spatial video frame resolution at thereceiver for time-bounded video transmitted over a communicationchannel. In particular, this is achieved by using a channel sensitivevideo encoder, determining the appropriate amount of channel bandwidthto reserve, utilizing the reserved bandwidth without wastage, anddeveloping a error-concealment mechanism for reconstructing the videoframe from incomplete data or data received in error, without the needfor re-transmissions.

In view of the many possible embodiments to which the principles of thisinvention may be applied, it should be recognized that the embodimentsdescribed herein with respect to the drawing figures is meant to beillustrative only and should not be taken as limiting the scope ofinvention. For example, those skilled in the art will recognize that theelements of the illustrated embodiments described in software andalgorithmic form may be implemented in hardware and vice versa or thatthe illustrated embodiments can be modified in arrangement and detailwithout departing from the spirit of the invention. Therefore, theinvention as described herein contemplates all such embodiments as maycome within the scope of the following claims and equivalents thereof.

What is claimed is:
 1. A method for reconstructing a video framereceived from a video source over a communication network, the methodcomprising the steps of: receiving from the video source a firstplurality of transmissions over the communications network wherein eachtransmission contains information representative of a discrete spatialcomponent corresponding to a particular region of a first video frame;receiving from the video source a second plurality of transmissions overthe communications network wherein each transmission containsinformation representative of a discrete spatial component correspondingto a particular region of a second video frame: determining if atransmission error is associated with any of the second plurality oftransmissions; substituting for each of the second plurality oftransmissions that is determined to have an associated error theinformation contained in one of the first plurality of transmissions tothereby form a reconstructed second video frame; and requesting that thevideo source give priority to a particular transmission in a thirdplurality of transmissions, wherein each transmission in the thirdplurality of transmissions contains information representative of adiscrete spatial component corresponding to a particular region of athird video frame, wherein the requested priority is a function of thedetermined errors associated with the second plurality of transmissions.2. The method as recited in claim 1 wherein the step of determining if atransmission error is associated with any of the second plurality oftransmissions further comprises the step of examining the content of thesecond plurality of transmissions.
 3. The method as recited in claim 1wherein the step of determining if a transmission error is associatedwith any of the second plurality of transmissions further comprises thestep of evaluating the timing of the second plurality of transmissions.4. The method as recited in claim 1 further comprising the step ofstoring in memory the information representative of the first videoframe.
 5. The method as recited in claim 4 further comprising the stepof replacing in memory the information representative of the first videoframe with the information representative of the reconstructed secondvideo frame.
 6. A computer-readable medium having computer executableinstructions for use in reconstructing a video image, the instructionsperforming steps comprising: receiving from a video source a firstplurality of transmissions over a communications network wherein eachtransmission contains information representative of a discrete spatialcomponent corresponding to a particular region of a first video frame;receiving from the video source a second plurality of transmissions overthe communications network wherein each transmission containsinformation representative of a discrete spatial component correspondingto a particular region of a second video frame; determining if atransmission error is associated with any of the second plurality oftransmissions; substituting for each of the second plurality oftransmissions that is determined to have an associated error theinformation contained in one of the first plurality of transmissions tothereby form a reconstructed second video frame; and requesting that thevideo source give priority to a particular transmission in a thirdplurality of transmissions, wherein each transmission in the thirdplurality of transmissions contains information representative of adiscrete spatial component corresponding to a particular region of athird video frame, wherein the requested priority is a function of thedetermined errors associated with the second plurality of transmissions.7. The computer-readable medium as recited in claim 6 wherein theinstructions for performing the step of determining if a transmissionerror is associated with any of the second plurality of transmissionsfurther comprise instructions for performing the step of examining thecontent of the second plurality of transmissions.
 8. Thecomputer-readable medium as recited in claim 6 wherein the instructionsfor performing the step of determining if a transmission error isassociated with any of the second plurality of transmissions furthercomprise instructions for performing the step of evaluating the timingof the second plurality of transmissions.
 9. The computer-readablemedium as recited in claim 7 further comprising instructions forperforming the step of storing in memory the information representativeof the first video frame.
 10. The computer-readable medium as recited inclaim 9 further comprising instructions for performing the step ofreplacing in memory the information representative of the first videoframe with the information representative of the reconstructed secondvideo frame.